Battery cells typically consist of a positive and negative electrode (cathode and anode) and a liquid electrolyte solution, separated by a thin, porous film known as a separator. A separator plays a key role in a battery. Its main function is to keep the two electrodes physically apart from each other in order to prevent an electrical short circuit. Accordingly, the separator should be electrically insulating. At the same time, the separator should allow rapid transport of ionic charge carriers that are needed to complete the circuit during cell charging and discharging. The separator should have the capability of conducting ions by either intrinsic ionic conduction (such as solid electrolytes) or by soaking the separator with a liquid electrolyte.
High temperature melt integrity (HTMI) of battery separators is a key property to ensure the safety of the battery pack. Specifically, high separator HTMI is important to provide an extra margin of safety. For example, in case the battery pack is subject to internal heat build-up from overcharging or internal short-circuiting, a separator with a high HTMI maintains its integrity (both shape and mechanical) and as a consequence, prevents the electrodes from contacting each other at high temperatures.
Lithium-ion batteries typically use separators made from polymers and, more specifically, polyethylene (PE) and polypropylene (PP), which are produced via melt processing techniques. These types of separators typically have insufficient melt integrity at high temperatures and are incompatible, i.e., non-wettable, with the electrolyte solutions. Therefore, a need exists for alternative separators with improved HTMI that can be produced via a melt or solution process.
Polyetherimides (for example Saudi Arabia Basic Industries Corporation's ULTEM branded polyethermide products) are attractive materials for battery separator applications because they combine outstanding characteristics, such as good electrolyte wettability, high solvent resistance, and HTMI typically exceeding 200° C. Polyphenylene oxides are also particularly suitable for HTMI battery separators, with HTMI values typically exceeding 200° C. Additionally, polyimides are also suitable to be used for HTMI separators, which are typically produced by processing poly(amic acid) into a desired form factor, followed by a heat treatment to form the polyimide. Alternatively, aromatic polyamides can be used as HTMI battery separators.
Conventional PP and PE separators are prepared by either the “dry process” or the “wet process”. Both processes rely on stretching, crystallization, and annealing of the polymers to generate the desired pore structure. Since polyetherimides, polyphenylene oxides, and poly(amic acids) (precursor to polyimides) are typically amorphous resins, these two conventional approaches are not suitable to produce polyetherimide, polyphenylene oxide, or polyimide-based separators. Additionally, the dry and wet processes lead to relatively low porosities and high tortuosity, which limits lithium-ion transfer through the separator, e.g., leading to relatively low power capability battery cells. Therefore, there exists a need for a membrane preparation process suitable for amorphous resins like polyetherimides, polyphenylene oxides, and poly(amic acids), where the process allows preparing porous structures meeting the requirements of battery separators.
In the case of lithium-ion batteries, polymeric separator films are typically based on PE and/or PP. The porosity is typically induced by uniaxial stretching of extruded films, which process is known as the “dry process” and is based on a complex interplay between extrusion, annealing, and stretching of the film (see e.g., U.S. Pat. Nos. 3,558,764 and 5,385,777). The “dry process” typically leads to an open pore structure and a relatively uniform pore size. However, inherent to the stretching process, the “dry process” leads to non-spherical pores and to residual stresses in the material. The latter typically leads to deformation (shrinkage) of the films over time, especially at elevated temperatures. Since crystallization/crystallinity is required during the stretching process in order to develop a porous structure, the preparation of porous films by the “dry process” is limited to semicrystalline polymers only. Although this process allows for a reasonably high porosity (30-50%), the actual accessible porosity (as measured e.g., by air permeability) is often lower, since not all pores are interconnected with each other.
Alternatively, porosity can be induced by pre-mixing the polymer with a low molecular weight extractable, which forms a specific structure upon cooling from the melt and, after removal of the low molecular weight species, leaves a porous structure (see e.g., U.S. Pat. No. 7,618,743, JP1988273651, JP1996064194, and JP1997259859). This process is known as the “wet process”, and typically uses a polymer/extractable combination that is miscible during the extrusion process, but phase separates upon cooling. Removal of the low molecular weight specie can be achieved by evaporation or extraction. An additional stretching (uniaxial or biaxial) step is sometimes used to create the desired pore structure. The “wet process” typically leads to a highly tortuous, interconnected porous structure. The preparation of porous films by the “wet process” is limited to polymers with a relatively high melt strength (e.g., ultra-high molecular weight PE). Also here the actual accessible porosity (as measured e.g., by air permeability) is often lower than the total porosity, since not all pores are interconnected with each other.
In all cases, high porosity of separator films is beneficial for the charging and discharging characteristics of batteries, since the volume resistivity of the cell typically scales inversely with the accessible separator porosity. Additionally, separator pore sizes need to be small enough to ensure it functions as an electrical barrier between the electrode, with pore sizes preferably smaller than the particle size of the anode and cathode active material (typically several micrometer). Also, the pore size distribution is preferably narrow and the pores are preferably uniformly distributed. Preferably, all pores are in some way connected from front to backside of the film or, in other words, the actual accessible porosity equals the total porosity. This means that all pores are accessible for the electrolyte solution and contribute to ion transport through the separator. In the case of lithium-ion batteries, high tortuosity and an interconnected pore structure is beneficial for long life batteries, since it suppresses the growth of lithium crystals on the graphite anode during fast charging or low temperature charging. On the other hand, an open (low tortuosity) and uniform pore size structure is beneficial for applications where fast charging and discharging is required, e.g., for high power density batteries.
Battery separators with a pore structure that is significantly more open than that of separators prepared via the “dry process” and “wet process” can be made via fiber spinning processes and organizing the spun fibers into woven or non-woven webs.
Polymers in the form of fibers are also useful in the applications of separators (electrolytical capacitors for example) or for substrates (fuel cell applications for example). Additionally, webs consisting of fibers, either with a sub-micron or supra-micron diameter, can be applied as medial implants, filtration membranes, dialysis membranes, water filtration membranes, desalination membranes, gas separation membranes, hospital gowns, electrical insulation paper and personal hygiene products. Also, webs comprised of polymer fibers can function as a substrate for further functionalization, e.g., by spinning other fibers onto the substrate, or by coating with other polymer or inorganic systems. Additionally, polymer fibers can be useful to functionalize substrates. An example could be to spin ultra-fine fibers onto a micro-porous web.
The conventional fiber fabrication technologies such as melt spinning, web spinning, dry spinning, or dry jet-wet spinning, comprise extrusion of a polymer melt or solution through a nozzle by a mechanical force followed by solidification of the melt or solution in order to fabricate fibers. These conventional fiber fabrication technologies typically produce fibers having a diameter ranging from several micrometers to several scores of micrometers. Consequently, the woven or nonwoven webs comprising such spun fibers typically contain pores too large to be applicable for lithium-ion battery separators, e.g., exceeding 5 μm, as the fiber diameter scales with the pore size of the web (see G. E. Simmonds et al., Journal of Engineered Fibers and Fabrics, 2(1), 2007). This large pore size would allow the particles of the anode and the cathode to migrate towards each other through the large pores to cause an internal short circuit. Additionally, the large fiber diameter makes it difficult to achieve thin separators, e.g., of 50 μm or less. For example, U.S. Pat. No. 5,202,178 describes melt spun polyamide with a fineness of 0.5-3.5 denier (fiber diameter about 8-20 μm), which are applicable as alkaline battery separators, but not as lithium-ion battery separators. Various methodologies to produce fine polymer fibers with a sub-micrometer average diameter have been described, such as in U.S. Pat. Nos. 4,044,404, 4,639,390, 4,842,505, 4,965,110, 5,522,879, and 6,106,913, where the formation of the fine fibers out of a polymer melt or a polymer solution typically relies on applying a pressure or an electro-static force. The latter method, commonly known as electro-spinning, is by far the most used technology to prepare fine fibers. Electro-spinning (comprising electro-blowing, melt-blowing, flash spinning or air-electro-spinning) is a technology known to be applicable to polymers of various forms, such as a polymer melt or a polymer solution, and the technology is able to produce fibers having a diameter of several nanometers up to thousands of nanometers. Such a small fiber diameter enables to produce polymer webs having a high porosity combined with a small pore size and provides new properties that are impossible to realize via the conventional fiber spinning technologies. Details around the electro-spinning method, setup, processing conditions and applications are widely described in literature, such as for example “Electrospinning Process and Applications of Electrospun Fibers” by Doshi and Reneker (J. Electrostatics, 35, 151-160 (1995)), “Electrospinning of Nanofibers in Textiles” by Haghi (CRC Press, Oct. 31 2011), “Beaded nanofibers formed during electrospinning” by H. Fong (Polymer, 40, 4585-4592 (1999)) and U.S. Pat. Nos. 6,616,435, 6,713,011, 7.083,854, and 7,134,857.
In the process for fabricating a porous polymer web using electro-spinning, a polymer solution is extruded through fine holes (e.g., a needle or nozzle) under an electric field to volatilize or solidify the solvent from the solution, which forms the fibers on the collector surface located at a predetermined distance. The polymer web thus obtained is a laminated three-dimensional network structure of fibers having a diameter of from several nanometers to several thousands of nanometers and has a large surface area per unit volume. Accordingly, the polymer web thereby obtained is typically superior in total porosity and reduced pore size to those produced by the other, conventional fabrication methods.
The main advantage of the electro-spinning process is that it enables to readily control the diameter of fibers in the polymer web, the total web thickness (i.e., from several micrometers to several thousands of micrometers) and the size of the pores by modifying the process conditions. The physical phenomenon that takes place when applying a high voltage to the liquid drops hanging on the orifice of e.g., a needle in the electro-spinning process is called “Taylor cone”. Here, a stream is formed to discharge the liquid drop towards the collector when the force of charges exceeds the surface tension of a solution to be suspended. An organic solution having a low molecular weight can be sprayed into fine liquid drops. However, due to its high viscosity and rheological characteristics, a polymer solution typically forms a stream that is split into several sub-streams with densely accumulated charges as it becomes apart from the Taylor cone to reduce the diameter. The large surface area of the polymer solution in the shape of fine streams accelerates solidification of the polymer solution and volatilization of the solvent, forming a polymer web with semi-entangled solid fibers on the surface of the collector.
Among the various parameters of the electro-spinning process are the applied voltage, the orifice to collector distance, the solution delivery rate, the polymer concentration, the viscosity, the solvent polarity, the surface tension of the solution, the solvent evaporation rate and the solution dielectric constant. A great increase in the discharged amount of liquid without adjusting the applied voltage accordingly will result in liquid drops being formed, rather than the desired nano-fibers, eventually leading to a polymer web in which fibers are mixed with liquid drops. A too high voltage makes the discharged polymer stream unstable and uncontrollable. A rise of the applied voltage or an increase in the discharged amount typically increases the diameter of the stream emitted from the Taylor cone to form a polymer with fibers having a larger diameter. It can be understood that finding the proper processing conditions for electro-spinning is, therefore, not straightforward, as e.g., described by Yao et al. (Yao et al., Journal of Membrane Science, 320(1-2), 2008, Pages 259-267). Additionally, the polymer needs to be well soluble in a solvent, where the combination of polymer/solvent needs to be suitable for the electro-spinning process (e.g., in dielectric constant, evaporation rate, viscosity, etc).
The electro-spinning process largely depends on the force of charges, which is a disadvantage in large-scale production over the conventional fiber fabrication processes, because the discharged amount from the nozzle is relatively small in production of a polymer web with fibers having a small diameter compared to the conventional processes. It is generally stated that the required time for the polymer solution to move from the orifice or nozzle to the collector and form solid fibers is significantly shorter than one second, normally 0.1 to 0.01 second. Assuming a typical orifice-nozzle distance of 10 cm, the fiber spinning speed is normally 1 to 10 m/s. Although the fiber spinning speed appears rather fast at first sight (1-10 m/s), it is important to understand that a single web of 0.1 m2 with a thickness of 50 μm and a total porosity of 50% consisting of fibers with a diameter below 1 μm has a total fiber length exceeding many hundreds of kilometers. So even at a spinning speed of 10 m/s, the electro-spinning process to prepare such a 0.1 m2 porous web typically leads to preparation times of several hours up to several days, which is not acceptable for large-scale, commercial nano-fiber web production. Varabhas et al. state that a 0.1 m2 nonwoven mat containing 1 g of 100 nm fibers may take several days to create from a single jet via an electro-spinning process (Varabhas et al., Polymer. 49(19), 2008, Pages 4226-4229). Many other sources state that electro-spinning is a very slow process, which severely limits its commercial value, for example Wertz et al., Filtration and Separation, 46(4), 2009, Pages 18-20; Ou et al., European Polymer Journal, 47(5), 2011, Pages 882-892; WO Patent Application 2008057426; von Locsecke et al., Filtration and Separation, 45(7), 2008, Pages 17-19. Additionally, the solvent handling and recovery in the electro-spinning process is intrinsically difficult (Ellison et al., Polymer, 48, 2007. Pages 3306-3316).
As discussed previously, electro-spinning production speeds cannot simply be improved by increasing the discharge rate out of the orifice, as this would typically result in the formation of liquid drops (defects) next to the (nano-)fibers. To increase the overall production speed of nano-fiber polymer webs, a plurality of needles, nozzles or orifices for discharging the polymer solution can be densely arranged, as for example described in Theron et al., Polymer. 46, 2005, Pages 2889-2899 or Lukas et al., Journal of Applied Physics, 103, 2008, 084309. Such a setup enables simultaneous spinning of multiple fibers, which increases the web production speed. However, even when 10 to 100 orifices would electro-spin nano-fibers simultaneously, the preparation of a 0.1 m2 nonwoven mat with a thickness of 50 μm and a total porosity of 50% consisting of fibers with a diameter below 1 μm will still take several hours, i.e., the process is still very time consuming. Additionally, as the orifices are typically densely arranged in a small space, it is more difficult to volatilize the solvent of the polymer solution. As a result, there is an increased possibility to form a polymer web having a film structure rather than a fiber structure, i.e., more defects will be present. This problem is a serious obstacle to high-speed or large-scale production of nano-fiber polymer webs using the electro-spinning process.
The application of the electro-spinning method to prepare nano-fiber webs for battery or capacitor separators has been explained in literature, e.g., WO Patent Application 2012043718 and U.S. Pat. Appl. No. 2002/0100725. Additionally. U.S. Pat. Appl. No. 2009/0122466 describes capacitor separators based on polyamide prepared via an electro-spinning process, where webs made out of nm-sized fibers were prepared by electro-blowing polyamide and depositing those directly on a moving collection belt, either in a single or multiple pass, after which the as-spun nano-web was dried by transportation through a solvent stripping zone with hot air and infrared radiation. The nano-webs were also calendared in order to impart the desired physical properties. U.S. Pat. No. 7,112,389 describes battery separators comprising a porous fine fiber layer of polyamide or polyvinyl alcohol fibers having a mean diameter of 50 to 3000 nm. The fine fibers are prepared via electro-blowing the polymer solutions. To improve the strength of the webs, the polyamide fine fiber web was thermally bonded, while the polyvinyl alcohol fine fiber web was cross-linked by a chemical procedure. U.S. Pat. No. 7,170,739 describes the application of such porous fine fiber layers of polyamide and polyvinyl alcohol for electrochemical double layer capacitors. U.S. Pat. Appl. No. 2011/0117416 describes that the electrolyte wettability of such fine fiber web separators can be improved by the introduction of a surfactant. JP Patent Application 2007211378 describes battery separators based on poly(4-methyl-1-pentene), where the polymer is shaped into the geometry of fibers with a diameter of 2 μm or less. KR Patent Application 2008013208 and 2010072532 and WO Patent Application 2011055967 describe heat-resistant, fine fibrous separators for secondary batteries, comprising a fibrous phase formed by electro-spinning or air-electro-spinning a heat-resistant polymer material (such as aromatic polyesters, polyimides, polyphenylene oxide, polyamide) in combination with a fibrous phase formed by electro-spinning consisting of a polymeric material that swells in the electrolyte solution (such as polyvinylidene fluoride, polyvinylchloride, PE oxide, polystyrene, polymethyl methacrylate). KR Patent Application 2008013209 describes a heat-resistant separator with a shutdown function for electrochemical devices used in, e.g., electric automobile, comprising an fine fibrous layer positioned on a porous substrate, where the fibrous phase is formed by electro-spinning a heat-resistant polymer (such as aromatic polyesters, polyimides, polyphenylene oxide, polyamide) and a polymer material that swells in the electrolyte solution (such as polyvinylidene fluoride, polyvinylchloride, PE oxide, polystyrene, polymethyl methacrylate). JP Patent 04963909 describes the production of fibrous battery separators based on polyphenylene oxide via an electro-spinning process, with average fiber diameters of 0.01-10 μm. Polymer fibers in the form of a woven or nonwoven web can also be used in laminated structures. JP Patent Application 2011077233 described the use of polyamide fibers of 10-600 nm in diameter prepared via an electro-spinning process, where the nano-fibers are spun on a fibrous support with fiber fineness of 0.01-5 dtex (about 1-25 μm average diameter). As described in U.S. Pat. Appl. No. 2012/0082884, the discussed electro-spinning process can be used to spin nano-fibers in a continuous fashion onto a substrate.
Therefore, there exists a need for a fiber preparation process that allows for the production of fine fibers at a throughput significantly higher than that of electro-spinning, and that allows for fiber diameters significantly smaller than those obtained from traditional melt-spinning techniques.
An alternative method to electro-spinning does not rely on an electro-static force to form the fine fibers from a single orifice, but rather on a centrifugal force. As the centrifugal force is the driving force for the formation of the fine fibers, the technology is generally known as force-spinning. U.S. Pat. Appl. Nos. 2009/0280207, 2009/0232920, 2009/0269429, and 2009/0280325 describe an apparatus that uses a rotating spinneret comprising an array of capillaries. This spinneret typically rotates at speeds from 500 to 25000 rpms, thereby creating a significant centrifugal force responsible for the formation of fine fibers. By increasing the number of capillaries in a given spinneret, the volumetric throughput of fiber generation can be increased to make more fibers in a short period of time. This technology can be applied to a polymer melt as well as to a polymer solution and has the advantage of having significantly higher throughputs as compared to the conventional nano-fiber spinning technology, such as electro-spinning. WO Patent Application 2012122485 describes the application of the described force-spinning method to prepare fine fiber of fluoropolymers having a contact angle greater than 150°. However, this technique has never been used to produce fibers based on high temperature materials, such as polyetherimides, polyphenylene oxides and poly(amic acids), which would be required for e.g., HTMI battery separators.
Another alternative to electro-spinning is a process whereby a polymer solution is injected through one or multiple small orifices into a non-solvent to the polymer, which, upon mixing of the solvent and non-solvent, induces precipitation of the polymer at a solvent/non-solvent composition at which the polymer is no longer soluble in the solvent/non-solvent mixture. When the non-solvent is sheared (e.g., flows) upon injection of the polymer solution, the precipitation of the polymer will occur under shear conditions, which enables the formation of fibers at very high throughput. As spinning of the fibers relies on the shear conditions of the non-solvent in which the polymer solution is injected, this process is known as shear-spinning. The fiber diameter is dependent on the process conditions. However, this technique has never been used to produce fibers based on high temperature materials, such as polyetherimides, polyphenylene oxides and poly(amic acids), which would be required for e.g., HTMI battery separators.
Therefore, there exists a need for a high throughput fiber production process based on mechanical spinning, shear spinning and/or electro-spinning that enables the production of fine fibers based on high temperature materials.